EAS systems of the type described above, are described in U.S. Pat. No. 3,665,449 (Elder and Wright). With such systems, a dual status marker of the type described above may be sensitized, i.e., the high-coercive force control elements thereof demagnetized, by applying an alternating, diminishing amplitude magnetic field, or by gradually removing an alternating field of constant intensity such as by withdrawing a bulk magnetic eraser of the type supplied by Nortronics Company, Inc. of Minneapolis, Minn. As disclosed in the U.S. Pat. No. 3,665,449 such a demagnetization operation may also be effected through the proper selection and arrangement of a series of permanent magnets in which adjacent magnets are oppositely polarized. By selecting the magnets to be of different strengths and by arranging them in an order ranging from highest to lowest (relative to the direction of travel), the magnetic field will appear to diminish in amplitude when passed over a control element. The patent also suggests that magnets of the same field strength may be arranged like inverted ascending steps or like an inclined plane so that the amplitude of the field is progressively diminished to produce the same result, and that it is not ordinarily necessary to demagnetize the control element in the strictest sense. Rather, the magnetic influence of the control element need only be reduced to an extent permitting magnetization reversal of the marker by the applied field.
While such techniques may be useful in many areas with the markers affixed to a wide variety of articles, the magnetic fields associated therewith have been found to unacceptably interfere with magnetic states associated with certain articles, such as prerecorded magnetic video and audio cassettes utilized in video rental businesses and in public libraries. Because of the compact size and popularity of such prerecorded magnetic cassettes, they are frequent targets for shoplifters, and hence likely articles with which anti-theft markers would be used. At the same time however, such affixed markers would be desirably sensitized upon return of the article, and it has been found that prior art demagnetization apparatus such as those described above may unacceptably affect signals prerecorded on the magnetic tapes within the cassettes.
In contrast to the demagnetization apparatus of the art described above in which the intensity of the magnetic fields produced thereby extend in a virtually uncontrolled fashion, the apparatus described in U.S. Pat. No. 4,689,590 (Heltemes) and U.S. Pat. No. 4,752,758 (Heltemes) provides a succession of fields of alternating polarity which rapidly decrease in intensity only a short, controlled distance from the surface of the apparatus and thus, while being capable of demagnetizing high-coercive force control elements of a marker brought close thereto, would be incapable of appreciably interfering with the magnetic signals recorded on tapes within a cassette to which the marker is affixed.
The Heltemes apparatus utilizes an elongated array of closely spaced poles whose field intensity is substantially similar but whose polarity alternates. This array is typically formed using a series of permanently magnetized elements made from the same material and having substantially similar dimensions. The array is positioned at an incline relative to a working surface such that a high-coercive force control element that is moved relative to the array along the working surface in the direction of increasing distance between the array and the working surface experiences a magnetic field that alternates in polarity and generally decreases in intensity. Used in such a manner, the apparatus causes the control element to become demagnetized. Demagnetization in such a manner is often referred to as "ring-down."
While the Heltemes apparatus is useful in demagnetizing control elements contained in anti-theft markers affixed to prerecorded magnetic tapes without affecting the signals prerecorded on such tapes, the array of alternating poles embodied therein is not designed for optimal ring-down. Optimal ring-down occurs when the alternating magnetic field decreases in an exponential envelope. The Heltemes apparatus relies on the gradually, and typically linearly, increasing distance between a working surface and a series of alternating poles of substantially similar strength to achieve decreasing field intensity at the working surface. Consequently, the length of the series of alternating poles in the Heltemes apparatus is significantly longer than necessary for an optimized magnetic array. In addition, because the first and last magnet elements in the array have only one neighboring magnet element of opposite polarity, the contributions from these end elements can be undesirably large, thus leaving the control element with a net magnetization. Correction of this problem also necessitates a longer array in the Heltemes apparatus.